Brunetting is the process of 'de-leaning,' so in essence going from being high, to being normal again. The time this process takes varies depending on how many joints you have smoked. This phrase was originally used in wanstead park by Alasdair Bain and Neil Dixon but is now more widely used.
'Holy fuck man, I am so so lean right now, I gotta brunette before I go home.....I no, we'll go somerfield then have another joint'
'FUCK ME I GOTTA BRUNETTE'
'Man brunetting's so enjoyable don't ya think?
I'm so high right now i don't no what the fuck's going on!'